Effect of sodium nitrite and regulatory mutations Δagr, ΔsarA, and ΔsigB on the mRNA and protein levels of staphylococcal enterotoxin D

Abstract

Staphylococcal food poisoning results from ingestion of enterotoxins produced by Staphylococcus aureus. Staphylococcal enterotoxin D (SED) is one of the most common toxins detected in S. aureus strains associated with intoxications. The effect of sodium nitrite on enterotoxin production has been only partly investigated, despite its wide usage in meat products. In addition, the factors influencing SED regulation are unclear. The aim of this study was to determine the effect of sodium nitrite on sed transcription and SED production, as well as the effect of regulatory mutations on SED protein levels. Temporal sed mRNA and SED protein levels were compared in LB and LB supplemented with 150 mg/L nitrite, and SED protein levels between wild type (wt) and isogenic regulatory mutants (Δagr, ΔsarA, ΔsigB) under control and sodium nitrite conditions. Relative sed mRNA levels of wt strains were higher in late stationary phase in the presence of nitrite compared to control conditions. However, SED protein levels were decreased in the presence of nitrite. In LB, Δagr mutants showed SED levels similar to the wt, while ΔsarA mutants exhibited reduced and ΔsigB mutants increased SED levels compared to the wt. In LB with sodium nitrite, SED levels of mutant strains were reduced similar to the wt strains, except for two Δagr mutants, in which SED levels were increased in the presence of nitrite. Overall, strain-specific variation with regard to the effect of regulatory mutations was observed. In addition, the data suggests that SED regulation may not be as tightly dependent on Agr as previously described.